Melt pour energetic compositions are currently made by blending various energetic ingredients with a low viscosity binder in a mixer. In order to meet the constant demand to improve the energy density, the conventional energetics manufacturing process has been pushed to the limit due to its inability to overcome its fundamental constraint, e.g., the “need to flow in order to mix”. As a result, energetic products at high solid loadings are often inhomogeneous, and, since the mixing quality performance is intractable, always require destructive testing methods for quality assurance. Furthermore, the ongoing efforts toward the increasing use of smaller energetic particles in order to enhance munitions insensitivity are also hindered by the limitation of conventional mixing equipment to handle highly viscous mixing media resulting from the enormous increase of particle surface area and the resulting dramatic viscosity increases of the “charge”.
The current art of manufacturing propellants and explosives usually involves mixing of the ingredients for long periods to achieve a uniform distribution of the solid ingredients in a viscous matrix that is still pourable but eventually slowly polymerizes or gels into a solid mass. Not only may the carrier fluid be viscous unto itself, but the slurry paste becomes very viscous because of the generally high loading of solids. In fact, the increase in viscosity caused by the solids loading frequently sets the maximum solids content that can be processed. Also, mixing of otherwise desirable small size particles causes an undesirable viscosity increase. The high viscosity of the mixture requires substantial torque to blend. The power required to mix, combined with long mixing times, result in inefficient mixing and high power consumption, leading to potentially undesirable and dangerous heat buildup. For example, mixing for the solid propellants such as those used in the space shuttle program typically contain the following ingredients: 12.6% by volume aluminum powder as fuel (10 to 20 microns); 63.5% ammonium perchlorate as oxidant, generally with a bimodal size distribution (about 20 microns and about 300 microns); 23.9% polymerizable mixture of prepolymer and plasticizer. Mixing the above formulation may be done in a planetary change-can mixer. A typical mix cycle might involve as long as 95 minutes or more from charging to cleanup. Power required is up to 100 HP for a 300 gallon batch. Attempting to mix faster can result in an unacceptable temperature rise in the batch.
Accordingly, there is a need for a process that avoids the energy consequences of attempting to uniformly mix a viscous slurry and the time consuming and potentially hazardous requirement to clean the mixer between batches. The proposed method allows for higher solids loading, achieves a high level of homogeneity and produces materials that are safer to handle.
The terms “ordered mixing” and “ordered mixtures” formed by such mixing processes were coined to describe the mixing of cohesive, interactive particulate systems, in differentiation to the traditional randomization mixing process of comparatively coarse, free-flowing, non-interacting particulate systems. Ordered Powder Mixing, Nature, Volume 262, Jul. 15, 1976, Pages 262-263, Chee Wai Yip, John A. Hersey; Ordered mixing: A new concept in powder mixing practice, Powder Technology, Volume 11, Issue 1, January-February 1975, Pages 41-44 J. A. Hersey. The basic principle of ordered mixing is that fine particles will adhere, especially to larger particles. The adhesional forces involved may be electrostatic, van der Waals, or surface tensional. Coarser components assist in the mixing process by breaking down agglomerates of the fine powder, thus allowing the adhesion of single cohesive particles to the surfaces of the coarser constituents.
Ideally, ordered mixtures in the form of clusters may be formed due to complete adhesion of an identical number of equal sized fine particles to each coarse, homosized carrier, resulting in perfect mixtures with zero standard deviation. It is not adhesion per se which allows the improved degree of homogeneity to be achieved, but only ordered adhesion. To produce ordered powder mixtures, it is the basic requirement that ordered, not random, adhesion must be achieved during mixing. To date, no prior art mechanism has been established that yields ordered adhesion.
Most work in ordered mixing is carried out in the pharmaceutical industry for production of solid dosage forms, where the small drug particles adhere on coarser excipient carrier particles to achieve content uniformity. In practice, ideal ordered mixtures with zero standard deviation have not been achievable, since the amount or number of particles interacting with carrier units is not constant but varies in such a way that can probably best be described as random. In the pharmaceutical industry, any addition of binder/glue in the ordered mixing process to enhance the adhesion between small drug particles and coarser carrier particles will reduce the dissolution rate and thus the bioavailability of the final dosage form. As a result, the relatively weak interactive forces between the guest (drug) and host (carrier excipient) particles tend to segregate by frictional attrition during subsequent solids handling and processing steps. Ordered mixtures formed by adhesion of particles will segregate into the two constituents if the forces applied to the mixture are sufficient to break the adhering bonds between particles. Constituent segregation takes place due to the frictional attrition, which causes fine particle dislodgement from the ordered mixture of clusters, between adjacent particles and the containing wall. Segregation can be as much a problem in an ordered mixture with weakly formed interaction as in a random mixture. An ordered system has a greater stability than a random system due to the particle interaction; but once this interaction is disturbed, the segregation pattern of an ordered mixture may be more unpredictable than a random mixture.
Although content uniformity is very important especially for very low content high potency drugs, no stoichiometry is required as is the case in energetics manufacture. It is believed that there is currently no process available for simultaneously coating fine as well as coarser particles and clustering particles to form an ordered mixture using a sequential particle addition process according to one embodiment of the present invention.